The present invention generally relates to gyro compasses and, more particularly, is directed to a high accuracy gyro compass for use with a ship.
2. Description of the Prior Art
Referring to the drawings in detail, and initially to FIG. 1, let us describe a gyro compass described in Japanese Patent No. 428317 as an example of a conventional gyro compass to which the present invention can be applied.
The entirety of the gyro compass is depicted by reference symbol A in FIG. 1, and the gyro compass A includes a gyro case 1. As shown in FIG. 1, the gyro case 1 houses therein a gyro rotor (not shown) which is rotated at high speed and at a constant revolution rate by an induction motor (not shown), and a rotary vector of the gyro rotor is directed to the south (i.e., directed in the clockwise direction as viewing from the north N). The gyro case 1 has a pair of vertical shafts 2, 2' protruded from the upper and lower portions thereof, and these protruded vertical shafts 2, 2' are respectively fitted into inner rings of ball bearings 4, 4' mounted to corresponding positions of a vertical ring 3 provided outside of the gyro case 1. A suspension wire 5 is secured at its lower end to the upper vertical shaft 2 and the upper end thereof is attached to the vertical ring 3 by means of a suspension wire mount 5'.
According to the above-mentioned arrangement, the weight of the gyro case 1 is not applied to the ball bearings 4, 4' of the vertical shafts 2, 2' as a thrust load but is fully received by the suspension wire 5, thereby the friction torque of the above-mentioned ball bearings 4, 4' being reduced considerably. A pair of liquid ballistics 6 are mounted on the east and west of the vertical ring 3 in order to apply a north-seeking torque to the gyro.
As shown in FIG. 2, each of the liquid ballistics 6 is a kind of a communicated tube and is composed of reservoirs 6-1', 6-1 disposed in the north and south of the gyro, liquid 6-2 of high specific gravity substantially filled into these reservoirs 6-1, 6-1' substantially up to the halves thereof, an air tube 6-3 communicating the north and south reservoirs 6-1', 6-1 above and a liquid tube 6-4 communicating the north and south reservoirs 6-1', 6-1 below.
Referring to FIG. 1, it will be seen that a damping weight 7 is mounted on the west side of the gyro case 1 in order to damp the north-seeking movement. As shown in FIG. 1, a primary coil 8-1 of a differential transformer for detecting a deviation angle between the gyro case 1 and the vertical ring 3 is attached to the east side of the gyro case 1, and a secondary coil 8-2 of the differential transformer is attached to the opposed position of the vertical ring 3, thereby constituting a follow-up pickup 8. The vertical ring 3 includes a pair of horizontal shafts 9, 9' protruded outwardly from the east and west positions thereof perpendicular to both of the vertical shafts 2, 2' and the gyro spin axis. These horizontal shafts 9, 9' are respectively fitted into inner rings of ball bearings 11, 11' attached to the corresponding positions of a horizontal ring 10 provided outside of the vertical ring 3. The horizontal ring 10 has a pair of gimbal shafts 12, 12' disposed at its positions within the horizontal plane and which are perpendicular to the horizontal shafts 9, 9' . These gimbal shafts 12, 12' are respectively fitted into a pair of gimbal shaft ball bearings 14, 14' attached to a follow-up ring 13 disposed outside of the horizontal ring 10.
As shown in FIG. 1, the follow-up ring 13 has upper and lower follow-up shafts 15, 15' and these follow-up shafts 15, 15' are respectively fitted into follow-up shaft ball bearings 17, 17' disposed at the opposing positions of a binnacle 16.
The upper follow-up shaft 15 has a compass card 18 attached at its shaft end and an azimuthal angle in the bow of a ship is read by the cooperation of the compass card 18 and a lubber line 18B secured to the binnacle 16 at the corresponding position in the bow side. An azimuth servo motor 19 is attached to the lower portion of the binnacle 16, the rotary shaft 19A of which is coupled through an azimuth pinion 20 to an azimuth gear 21 located at the lower portion of the follow-up ring 13. An azimuth transmitter 22 is attached to the lower portion of the binnacle 16 and its rotary shaft 22A is meshed with the azimuth gear 21 via a gear train (not shown), whereby an azimuth signal is converted into an electrical signal by the azimuth transmitter 22, which is transmitted to the outside.
The part within the horizontal ring 10, that is, the part including the horizontal ring 10, the vertical ring 3, the gyro case 1 or the like is normally called a gyro sensitive element. The gyro sensitive element constructs a vertical physical pendulum around the gimbal shafts 12, 12', whereby the horizontal shafts 9, 9' are constantly kept within the horizontal plane regardless of ship's inclination.
If there is a difference between the azimuth of the gyro case 1 and the azimuth of the vertical ring 3, then such difference is detected and converted into an electrical signal by the follow-up pickup 8 provided between the gyro case 1 and the vertical ring 3. The resultant electrical signal is amplified by an external servo amplifier 23 and supplied to the azimuth servo motor 19 (which forms an azimuth servo system). The rotation of the azimuth servo motor 19 is transmitted through the rotary shaft 19A, the gear train (not shown) and the azimuth gear 21 to the follow-up ring 13 and is further transmitted through the horizontal ring 10, the horizontal shafts 9, 9' or the like to the vertical ring 3, thereby constantly holding the azimuthal error between the vertical ring 3 and the gyro case 1 at zero.
Owing to the action of the azimuth servo system, the horizontal shafts 9, 9' and the gyro spin axis are constantly kept in an orthogonal relation and the gyro can be prevented from being applied with the twisting torque of the suspension wire 5. That is, owing to the actions of the three shafts such as the vertical shafts 2, 2', the horizontal shafts 9, 9' and the gimbal shafts 12, 12' having the servo system, the gyro case 1 is completely isolated from the angular motion of the ship, thereby the gyroscope being constructed.
The above-mentioned liquid ballistics 6 are adapted to give the gyroscope the north-seeking force, i.e., function as the compass.
The principle of the liquid ballistic 6 will be described with reference to FIG. 2. FIG. 2 shows the case such that the north-seeking end of the gyro is inclined upward from the horizontal plane by an angle .theta.. In this case, assuming that the ship is in its stopped condition, then the liquid surface of the liquid 6-2 becomes perpendicular to the direction of gravity force g. Therefore, as compared with the case such that the inclination of the north-seeking end relative to the horizontal plane is zero, the liquid in the hatched portion of FIG. 2 is decreased in the north-side reservoir 6-1' and is increased in the south-side reservoir 6-1. Assuming now that r.sub.1 is a distance from the horizontal shafts 9, 9' to the center of the two reservoirs 6-1, 6-1', S is a cross section area of the two reservoirs 6-1, 6-1' and .rho. is a specific gravity of the liquid 6-2, then the weight W of the liquid in the inclined portion is expressed as: EQU W=S.times.r.sub.1 sin .theta..times..rho..times.g (1)
Since the above-mentioned weight unbalance occurs in the two south and north reservoirs 6-1, 6-1' and the moment arm from the horizontal shafts 9, 9' is r.sub.1, a torque T.sub.H produced about the horizontal shafts 9, 9' by the liquid ballistics 6 when the north-seeking end of the gyro is inclined from the horizontal plane by .theta. is approximately calculated as: EQU T.sub.H =2 S r.sub.1.sup.2 g .rho..theta. (2)
When the following equation (3) is established, EQU 2Sr.sub.1.sup.2 g.rho.=K (3)
K is referred to as the ballistic constant. That is, the liquid ballistics 6 act to apply the torque proportional to the inclination of the gyro spin axis relative to the horizontal plane to the tyro around the horizontal shafts 9, 9' of the gyro, thereby rendering the north-seeking force to the gyro. Thus, the gyro is rendered the gyro compass.
As described above, we have considered so far the case that the ship is in the still condition. In this case, assuming that .alpha..sub.N is a south-north component of ship's acceleration due to increase and decrease of ship's speed, ship's turning or the like, a torque T.sub.H1 generated from the liquid ballistic 6 under the ship's sailing condition is expressed by the following equation: ##EQU1##
As shown in FIG. 3, the damping weight 7 is attached to the gyro case 1 with a distance r.sub.2 (in the direction perpendicular to the sheet of drawing) from the vertical shafts 2, 2' within the plane including the vertical shafts 2, 2' and perpendicular to the gyro spin axis. FIG. 3 shows the gyro case 1 under the condition such that the north-seeking side of the gyro is inclined upward from the horizontal plane by the angle .theta. as viewing from the west. As shown in FIG. 3, a gravitational acceleration g acts on the damping weight 7 of mass m so that a force of m.times.g acts on the damping weight 7 in the vertical direction. In this case, let us consider that this force is divided into a component m g cos .theta. parallel to the vertical shafts 2, 2' and a component m g sin .theta. parallel to the spin axis. The component m g cos .theta. parallel to the vertical shafts 2, 2' acts only as a load on the vertical shaft ball bearings 4, 4', while the component m g sin .theta. parallel to the spin axis acts on the gyro as a torque multiplied with a distance r.sub.2 from the vertical shafts 2, 2' around the vertical shafts 2, 2'. Assuming that T.phi. represents the above torque, then the torque T.phi. is approximately given by the following equation: EQU T.phi.=.mu..multidot..theta. (5)
where .mu.=m g r.sub.2.
That is, the damping weight 7 can be regarded as the apparatus which applies the vertical axes 2, 2' of the gyro with the torque proportional to the inclination of the gyro spin axis relative to the horizontal plane, and the north-seeking motion of the compass can be damped by the damping weight 7.
Further, a torque T.phi.1 generated during the ship's sailing is expressed by the following equation, considering the acceleration caused by ship's motion: ##EQU2##
FIG. 4 is a schematic functional block diagram which shows the motion of the spin axis of the gyro compass according to the prior art by the Laplace operator and the transfer function in which an azimuth error .phi. of the north-seeking end of the gyro spin axis from the true north and the inclination angle .phi. are taken as variants. In FIG. 4, g represents the gravitational acceleration, R the earth radius, .OMEGA. the rotation angular velocity of earth, H the angular momentum of gyro, .lambda. the latitude at that spot, .tau.G the time constant provided when the movement of the liquid surface of the ballistic 6 is approximated by the primary delay, K the north-seeking constant, .mu. the damping constant, .alpha..sub.N the acceleration acting on the south-north direction of the gyro case due to the ship's movement, V.sub.NS the north-south speed of the ship and S the Laplace operator.
A sum (difference) of the gyro inclined angle .theta. and a value .alpha.N/g, which results from dividing the south-north acceleration .alpha.N by the gravitational acceleration g, acts on the primary delay transfer element 50 (time constant .tau.G) provided by the liquid 6-2 of the ballistic 6 to form the liquid surface inclination .xi..
A precessional angular velocity ##EQU3## provided by multiplying .xi. with a value K/H (51), which results from dividing the north-seeking constant K by the angular momentum H of the gyro and which is generated around the vertical axis acts around the vertical axis of the gyro case 1 (52) together with the vertical component .OMEGA. sin .lambda. of the earth rotation angular velocity .OMEGA. to produce the azimuthal movement around the vertical axis. Then, the azimuth error .phi. is generated. A value, which results from multiplying the azimuth error .phi. with the horizontal component .OMEGA. cos .lambda. 53 of the earth rotation angular velocity .OMEGA., is input to a gyro element 54 around the horizontal axis of the gyro as the angular velocity input to thereby generate the gyro inclined angle .theta..
The above-mentioned portion is what might be called a north-seeking loop of the gyro compass, in which two poles expressed by 1/S exist within the loop, thereby generating the oscillation solution.
An angular velocity ##EQU4## which results from dividing by the gyro angular momentum H the torque ##EQU5## around the vertical axis in which ##EQU6## which results adding the gyro inclined angle ##EQU7## is multiplied with the damping constant .mu. is input to the gyro element 54 around the horizontal axis together with the equivalent angular velocity V.sub.N /R which results from dividing the south-north speed V.sub.N of the ship by the earth radius R, whereby the gyro inclined angle .theta. is reduced and the north-seeking movement is damped. Therefore, this loop is called a damping loop.
For the north-seeking loop, the south-north speed V.sub.N generates an azimuth error .nu. proportional to the second of the latitude expressed by the following equation. ##EQU8## where C is the azimuth angle of the ship's heading.
FIG. 5 is a graph illustrating the movement of the gyro when the ship turns by 180.degree. at time t.sub.1 from the condition that the ship sails straight ahead on the course 0.degree. for a long period of time and the gyro compass is settled with speed error .phi..nu..sub.1 at that time and then the ship sails straight ahead on the course 180.degree. from time t.sub.2. This fundamental influence of the gyro compass exerted by the acceleration can be reduced to the general case of the ship's movement.
An azimuth change .phi..sub.8 generated by the acceleration between the time t.sub.1 and the time t.sub.2 is called as ballistic angle. A design method for making the azimuth change .phi..sub.8 equal to the difference between the speed errors before and after the acceleration acts is the important condition called the Schuler tuning in the gyro compass and corrects the influence of the acceleration in the form of acceleration error (the north-seeking period of the gyro compass is extended to 1 to 1.5 hours due to this condition). That is, EQU .phi..sub.8 =.phi..sub..nu.1 -.phi..sub..nu.2 ( 8)
The above-mentioned ballistic amount .phi..sub.8 is the function of the speed difference and the difference of the speed error is also the function of the latitude as expressed in the above-mentioned equation. Therefore, strictly speaking, the condition of the above-mentioned equation is established only at a particular latitude (referred to as a reference latitude). In other latitudes, an error .DELTA..phi. of FIG. 5 is generated immediately after the ship turns and then in accordance with the fundamental movement characteristics of the gyro compass, the gyro compass carries out the damping movement toward the velocity error .phi..nu..sub.2 provided immediately after the ship turns.
The conventional gyro compass produces an error by the acceleration which is caused by the turning of the ship, its speed changing and so on. However, when the integrated value of the above error during the movement such as the turning, speed changing and so on is made just equal to the changing value of the acceleration error, that is, it is corrected by employing the Schuler tuning method, the acceleration error can be suppressed to such a value which causes no problem in practical use. However, recently the speed of a ship becomes higher, and a new high speed ship such as a hydrofoil craft or the like has been developed so that the error correction by using the Schuler tuning proposes a problem of acceleration error in practice.
The period of north-seeking movement of a gyro compass is changing depending on a latitude where the gyro compass exists, while the Schuler tuning is, in strictly speaking, established on the condition that the north-seeking movement period is always 84.5 minutes. Therefore, even if the Schuler tuning is established at a certain latitude for a gyro compass, when the gyro compass moves to another latitude, the Schuler tuning becomes no more established and hence an acceleration error is produced.
When the speed of a ship is about 10 to 15 knots, as in the prior art, if a reference latitude is set at any angle between latitudes of 50.degree. and 55.degree. and the Schuler tuning is established at that angle of latitude, an acceleration error can be suppressed to a tolerable value within a range between 65.degree. and 70.degree. from the equator. However, the above error correction method can never be established for a ship with a speed of 20 to 50 knots. Further, this method is not satisfactory for a ship, which is low in speed but requires high accuracy for its gyro compass, to cause a problem because its accuracy is not satisfactory.